1. Field
The invention relates to an agitator suitable for use in bioreactors.
2. Description
Agitators in bioreactors are essentially used to equalize temperature differences and differences of concentrations of various constituents. Agitators intensify the heat exchange between the thermostatting elements and the fermentation broth. Agitators also prevent the cells from sedimenting and thus being inhomogeneously distributed during the fermentation. Another task of agitators is to disperse the gas phase in the fermentation broth.
In biotechnology, various types of agitators are used. The most frequently used agitator is the disc agitator. A standard disc agitator is, for example, the Rushton turbine having six perpendicularly arranged blades. Such an agitator generates a flow radial to the agitator axis. Above and beneath such an agitator, flow vortices are formed, which result in a high dispersion effect. A second agitator is the inclined-blade agitator. This is an agitator where the angle of attack of the blades (with respect to the agitator axis) while changeable is usually 45°. The inclined-blade agitator has primarily an axial transport direction with a radial component and therefore, achieves a highly effective mixing. One disadvantage of disc agitators or inclined-blade agitators is that such agitators are easily flooded especially in the case of high gas load. As a result, the disc agitators are no longer able to disperse the exiting gas completely. A third type of agitator, the propeller agitator, is used in cell fermentation, but to a far lesser extent than standard disc agitators or inclined-blade agitators. The flow in a propeller agitator is directed axially.
Fundamentals of agitator technology in bioreactors are disclosed, for example, in Riet, van't, Tramper, J., Basic Bioreactor Design, Chapter 4: Kinetics, Marcel Dekker Inc., 1991; Tatterson, G. B., Fluid Mixing and Gas Dispersion in Agitated Tanks, McGraw-Hill, Inc., 1991; and Bailey, J. E. and Ollis, D. F., Biochemical Engineering Fundamentals, Second Edition, McGraw-Hill, Inc., 1986. In EP 0 745 666, a bioreactor is disclosed that is equipped with disc agitators. Disc agitators are also disclosed in DE 23 49 106 and DE 23 51 763.
U.S. Pat. No. 4,468,130 discloses an inclined-blade agitator having cambered agitator blades, the angle of which changes about 16° over the agitator blade. The angle at the tip of the agitator blade is between 16° and 32°. U.S. Pat. No. 4,896,971 discloses an inclined-blade agitator having twisted agitator blades, with twisting between 8° to 12°. The angle at the tip of the agitator blade is between 18° and 34°.
U.S. Pat. No. 5,052,892 discloses an agitator having agitator blades which are bent once at the center, with the angle between the two parts of the agitator blades being about 20° to 30°. U.S. Pat. No. 5,297,938 likewise discloses an agitator with bent agitator blades, and the angle between the two parts from 7.5° to 22.5°. U.S. Pat. No. 5,316,443 discloses an agitator having a hook-shaped cross section. U.S. Pat. No. 5,326,226 discloses an agitator having twisted agitator blades with the angle of the agitator blades between 25° and 45°.
U.S. Pat. No. 5,791,780 discloses an agitator in which the agitator blades have a semicircular or semi-elliptical cross section, while FR 1 600 744 discloses an agitator in which the agitator blades have a mainly triangular cross section. DE 94 00 938 U discloses an agitator in which the agitator blades have an inner main blade connected with an outer side blade.